136 GEORGE C. MATSPN 



two ends of Cayuga Valley is due to a difference in rock texture/ 

 The southern part of Cayuga Valley is cut in the hard, durable 

 Portage sandstone, while farther north are the soft Hamilton 

 shales which break down much more easily when exposed to the 

 agents of weathering. Another point of considerable, though 

 undetermined, importance in this connection is the intense glacia- 

 tion to which the northern end of the lake valley has been sub- 

 jected. As suggested by Dr. G. K. Gilbert,^ the belt in which 

 the northern end of the lake lies has been much more intensely 

 scoured by the ice than the belt south of the lake. The width 

 of the lake varies from about a mile near its southern end to 

 about three miles near Aurora. The surface is 378 feet A. 

 T., and the greatest depth is 432 feet. For considerably 

 more than half its length the bottom of Cayuga Lake is below 

 sea level. The drainage of Cayuga Valley is northward into 

 Lake Ontario. 



Direction of preglacial drainage. — While some of the earlier 

 writers believed that the preglacial river which occupied Cavuga 

 Valley flowed southward, all the later students are agreed that it 

 drained northward into the Ontario basin. The hypothesis of a 

 northward preglacial drainage is based on the following lines of 

 evidence: the general northward slope of the land; the wider 

 and more mature aspect of the valley as one passes northward 

 from the divide ; the increasing number of tributaries near the 

 divide ; and the fact that the bottoms of the mature tributaries 

 become lower from the divide northward. While all these lines of 

 evidence have been affected by the various changes accompany- 

 ing glaciation, it has not been sufificient to destroy their value as 

 evidence in this connection. Since there are no facts opposed 

 to the theory of a preglacial drainage toward the north, we may 

 regard the hypothesis as established. 



Postglacial gorges. — With few exceptions, all the tributary 

 streams which occupy mature preglacial valleys enter the main 

 valley through narrow, rock-walled, postglacial gorges contain- 



'^ New York Geological Survey of the Fourth District (1843), p. 225 ; Monograph 

 XLI, U. S. Geological Survey, p. 79. 



^ Paper read before the Geological Society of America, December, 1902. 



